Regulator for a variable displacement pump

Abstract: A regulator (10) for a variable displacement pump is provided with a hydraulic servo controller (11) capable of controlling the displacement volume of the pump and accomplishing a feedback function depending upon the working pressure (Pd₁) from the pump, an effective spring force and a pilot pressure (Pc).

Description: Field of the Invention The present invention relates to a regulator for a variable displacement pump; and, more particularly, to a regulator provided with a hydraulic servo controller capable of controlling the displacement volume of the pump and accomplishing a feedback function depending upon the working pressure from the pump, an effective spring force and a pilot pressure to achieve a stable flow rate and reduce energy losses. Description of the Prior Art A conventional variable displacement pump comprises a regulator which employs a device for controlling the angle of a swash plate therein. For example, Japanese Laid-Open Publication No. 89-116294 discloses a regulator comprising: a valve body; means for controlling the angle of a swash plate; a directional changeover valve defining a position for supplying a working pressure to a first chamber, a position for sealing the first chamber and a position for discharging the load pressure from the first chamber; a feedback lever for connecting an end of the changeover valve with a servo-piston of the control means; a first pilot spool movable by the working pressure from the pump for controlling the horsepower thereof; a second pilot spool movable by the pilot pressure, wherein a reservoir pressure is included, for controlling the flow rate of the pump; and a linkage mechanism for selectively applying one of the first and the second pilot spools to the feedback lever to reduce the discharge flow rate thereof. However, the linkage mechanism of the regulator complicates the overall structure and construction thereof. Further, since the flow rate of the pump is likely to be affected by a variation in the reservoir pressure of the hydraulic system during the flow rate control, it is difficult to achieve and maintain a predetermined target flow rate. In addition, the regulator is handicapped by the absence of a flow rate cutoff control and a flow rate cutoff release control, thereby causing an energy loss due to its inability to prevent an unnecessary discharge flow. Summary of the Invention Accordingly, it is an object of the present invention to provide a regulator for a variable displacement pump having a simplified structure by eliminating the linkage mechanism. It is another object of the present invention to provide a regulator for a variable displacement pump capable of accomplishing a stable flow rate control by eliminating the effect of the reservoir pressure of the hydraulic system. It is still another object of the present invention to provide a regulator for a variable displacement pump capable of reducing an energy loss by providing a flow rate cutoff control when the working pressure is greater than a predetermined value. The above and other objects of the present invention are accomplished by means of a variable displacement pump incorporating therein a regulator for controlling the flow rate of a fluid therein by means of varying the angle of a swash plate, which comprises: a pump for producing a flow of the fluid; a valve body in fluid-communication with the pump; a servo-piston pivotally mounted to the swash plate and provided with two portions, each having a different diameter, which are movable along a first and a second chambers formed within the valve body; a multi-stage piston movable along a first and a second piston compartments in the valve body by a working pressure from the pump; a control rod movable along an axis of the servo-piston and having a servo-spool for changing the direction of the flow; a servo-sleeve in fluid-communication with the servo spool for defining a neutral position, a position for supplying the working pressure of the second chamber and a position for discharging the working pressure to a reservoir; a cut-off spool in fluid-communication with the servo sleeve for defining a position for supplying the working pressure from the pump to the second chamber and a position for discharging the working pressure from the second chamber to the servo sleeve; and means for biasing the control rod and the servo-piston to be spaced apart from each other. Brief Description of the Drawings The above and other objects, advantages and features of the present invention will be apparent from the following description of preferred embodiments taken in conjunction with the accompanying drawings, wherein: Fig. 1 is a circuit diagram of a regulator for a variable displacement pump in accordance with the present invention; Fig. 2 is a cross-sectional view of one embodiment of the regulator in accordance with the present invention; Fig. 3 is a graph showing the relationship between the flow rate and the working pressure; and Fig. 4 is a graph illustrating the relationship between the flow rate and the pilot pressure. Detailed Description of the Preferred Embodiments Referring to Figs. 1 and 2, there is shown a regulator 10 in accordance with the present invention. A pair of variable displacement pumps Pd 1 and Pd 2 driven by an engine 12 produce a flow rate for the hydraulic pumps Pd 1 , Pd 2, which is controlled by means of varying the angle of a swash plate 14. A gear pump Pg is also driven by the engine 12 to deliver a pilot pressure to the regulator 10. A valve body 11 is installed to be fluid communicable with the pumps Pd 1 ,Pd 2 and Pg. The swash plate 14 is pivotally mounted to a servo-piston 16 at its one portion of the periphery thereof by a connecting rod 18 fixed to the servo-piston 16. The servo-piston 16 has two portions: i.e., portion a having a relatively small diameter and portion b having a relatively large diameter, each of which is slidably movable along a first chamber 20 and a second chamber 22 formed within the valve body 11. The first chamber 20 is fluid communicable with the pump Pd 1 while the second chamber 22 is fluid communicable with a reservoir 24 through a cutoff spool 26 and a servo-spool 28 having a changeover recess 30. A servo-sleeve 34 has three holes 31, 32, 33 for defining a neutral position as shown in Fig. 2, a position for supplying the working pressure to the second chamber 22 via the cut off spool 26 and a position for discharging the working pressure to the reservoir 24. A control rod 36 is connected to the servo-piston 16 by a first and a second springs 38, 40 capable of transmitting to the servo spool 28 the difference between the forces acted on the control rod 36 and the servo-piston 16, thereby changing or adjusting the hole position of the servo-sleeve 34. A multi-stage piston 42 in contact with the control rod 36 is slidably located in a first and a second piston compartments 44, 46, having a plurality of stages, each of which has a different diameter. As shown in Fig. 2, the control rod 36, the servo-piston 16, the multi-stage piston 42, and the springs 38,40 and 66 have a coaxial relationship with one another. The working pressure from the pumps Pd 1 , Pd 2 is supplied to the first and the second piston compartments 44, 46 through pressure lines 48, 50, and a pilot pressure is applied to a pilot chamber 52, a drain chamber 54 and a third piston chamber 56 through pilot lines 58, 60 from a control valve 64 and a line 62 through a reducing valve 76 from the gear pump Pg. The gear pump Pg is installed to maintain a constant initial pressure of the hydraulic circuitry to thereby prevent a delayed response time of the regulator 10 at a low working pressure. Thus, the working pressure initially supplied to the first chamber 20 is the higher one of the working pressure of the pump Pd 1 and the gear pump Pg. Hereinafter, how the regulator operates, depending upon the working pressure applied to each of the circuit elements, will be described with reference to the accompanying drawings. First, when the working pressure from the pumps is lower than the recovering force of the third spring 66, the movement toward the left side of the servo-spool 28 causes the second chamber 22 to be fluid communicated with the reservoir 24. Therefore, since the working pressure applied in the first chamber 20 is greater than that in the second chamber 22, the movement to the right side of the servo-piston 16 increases the angle of the swash plate 14 so that the discharge flow rate from the pumps may reach up to its maximum level. On the other hand, when the summation of the working pressures from the pumps Pd 1 , Pd 2 is higher than point A shown in Fig. 3, the working pressures supplied to the first and the second piston compartments 44, 46 defeats the recovering force of the springs 38, 66, thereby moving the control rod 36 to the right. At the same time, the cut off spool 26 also moves to the right by the action of the working pressure from a line 68 to have the holes 31 and 33 communicate each other. Thus, the working pressure is evenly provided in the first and the second chambers 20 and 22. However, since the hydraulically pressurized area of the servo-piston 16 located in the second chamber 22 is broader than that located in the first chamber 20, the movement to the left side of the servo-piston 16 decreases the angle of the swash plate 14 to thereby reduce the dicharge flow rate from the pumps and tends to urge the outer spring 38 against the control rod 36. Consequently, the control rod 36 is moved to the left to reduce the area, through which the flow is passed, formed by the relative position of the holes 31,32 and 33 of the servo-sleeve 34 and the recess 30 of the servo-spool 28, thereby decreasing the working pressure supplied to the second chamber 22. When the working pressure in the second chamber 22 becomes lower than that in the first chamber 20 again, the servo-piston 16 is moved to the right and the control rod 36 is also moved to the right due to a relaxation of the spring 38. Accordingly, the reciprocating movement of the control rod 36 does not stop until the resultant pressure balance among the working pressures in the chambers 20, 22, 44, 46 and the recovering force of the springs 38, 66 reaches an equilibrium, thereby controlling the horsepower of the pumps at a constant level. Further, when the summation of the working pressures from the pumps Pd1 and Pd2 is greater than the point B shown in Fig. 3, the control rod 36 becomes urged against the springs 38 and 40 so that the springs 38 and 40 are coincidently compressed to form a transition point B on the slope shown in Fig. 3. Thus, an increase of the working pressure causes a decrease of the discharge flow rate of the pumps. The operational process of the section B-C is similar to that of the section A-B as mentioned above except that its slope is different from the slope of the section A-B. Furthermore, when the working pressure becomes greater than point C shown in Fig. 3, the working pressure supplied to a first cutoff piston compartment 70 overcomes the resilient force of a fourth spring 72 so that the cutoff spool 26 is moved to directly supply the working pressure into the second chamber 22. Thus, the servo-piston 16 is moved to reduce the angle of the swash plate 14, thereby reducing the discharge flow rate of the pumps up to its minimum value, i.e., point D shown in Fig. 3. The flow rate cutoff control, therefore minimizes the energy loss by reducing the discharge flow rate up to the minimum value. In contrast, when the variable displacement pump requires the flow rate more than that of the section C-D, a pilot pressure Pcf from the exterior(shown in dotted lines) is supplied into a second cutoff piston compartments 74. Thus, the recovering force of the fourth spring 72 added to the pilot pressure Pcf in the second cutoff piston compartment 74 defeats the working pressure in the first cutoff piston compartment 70 to ensure that the second chamber 22 is communicated with the reservoir 24 through the hole 31 of the servo spool 28 via the cut off spool 26, although the working pressure higher than the point C is supplied therein. Accordingly, the servo-piston 16 is moved to increase the discharge flow rate of the pumps Pd1 and Pd2 more than their minimum flow rate, thereby releasing the flow rate cutoff control. Fig. 4 illustrates a graph showing the relationship between the pilot pressure and the flow rate. First, referring to Fig. 1, the pilot pressure Pc from the control valve 64 is directly supplied to the pilot chamber 52 through the line 58 and the pilot pressure Pcf is supplied to the drain chamber 54 via an orifice 65 through a line 60. Thus, the pilot pressure Pc has a higher value than that of the pilot pressure Pcf. Accordingly, the flow rate control is accomplished by the pressure difference \P between the pressure Pc in the pilot chamber 52 and the pressure Pcd of the drain chamber 54 as shown in Fig. 4, wherein its operational process is similar that of Fig. 3. Turning now to Fig. 3, there is shown another horsepower control diagram, i.e., line A''-B''-C'', accomplished by a pilot pressure Pps from the gear pump Pg. The pilot pressure Pps from is supplied into the third piston compartment 56 to increase the working pressure applied to the multi-stage piston 42. The resultant force acted on the multistage piston 42 urges the control rod 36 so that the discharge flow rate begins to be reduced to control the horsepower constantly along the line A''-B''-C'' shown in Fig. 3, when the working pressure is lower than the point A'', wherein its operational process is similar to that of the line A-B-C shown in Fig. 3. Although the invention has been shown and described with respect to the exemplary embodiments, it should be understood by those skilled in the art that various changes, modifications and additions may be made, without departing from the scope of the invention.

Claims: A variable displacement pump incorporating therein a regulator for controlling t-he flow rate of a fluid therein by means of varying the angle of a swash plate, which comprises: a pump for producing a flow of the fluid; a valve body (11) in fluid-communication with the pump; a servo-piston (16) pivotally mounted to the swash plate and provided with two portions, each having a different diameter, which are movable along a first and a second chambers (20, 22) formed within the valve body; a multi-stage piston (42) movable along a first and a second piston compartments (44, 46) in the valve body (11) by a working pressure from the pump; a control rod (36) movable along an axis of the servo-piston (16) and having a servo-spool (28) for changing the direction of the flow; a servo-sleeve (34) in fluid-communication with the servo spool (28) for defining a neutral position, a position for supplying the working pressure of the second chamber (22) and a position for discharging the working pressure to a reservoir; a cut-off spool (26) in fluid-communication with the servo sleeve (34) for defining a position for supplying the working pressure from the pump to the second chamber and a position for discharging the working pressure from the second chamber to the servo sleeve (34); and means (38, 40) for biasing the control rod and the servo-piston to be spaced apart from each other. The variable displacement pump as recited in claim 1, wherein said variable displacement pump further comprises a control valve (64) for supplying a pilot pressure to a pilot chamber (52) in which the control rod (36) is located, and a drain chamber. The variable displacement pump as recited in claim 1, wherein said cutoff spool (26) has a cutoff piston, movable along a first and a second cutoff piston compartments (70, 74) communicating with the pump and a pilot line. The variable displacement pump recited in claim 1, wherein said biasing means has an outer spring (38) located between the control rod and the servo-piston and an inner spring (40) having a smaller diameter and a shorter length than those of the outer spring, which is located within the outer spring. The variable displacement pump as recited in claim 1, wherein said pilot pressure from the gear pump (Pg) is supplied via a reducing valve (76) to a third piston compartment (56) formed with in the valve body to increase the working pressure applied to the multi-stage piston. The variable displacement pump as recited in claims 1 and 4, wherein said control rod (36), said servo-piston (16), said multi-stage piston (42) and said springs (38, 40) have a coaxial relationship with one another.

Inventors: KIM TAE JONG, KIM YOUNG GUK, KIM, TAE JONG, KIM, YOUNG GUK
Applicants: DAEWOO HEAVY IND CO LTD, DAEWOO HEAVY INDUSTRIES CO. LTD.
Application year: 1994
Main classification: F04B1
IPC codes: F04B1, F04B49